1. Field of the Invention
The present invention relates to an apparatus for controlling the amount of slip of a lock-up clutch provided in a power transmitting system of a motor vehicle.
2. Discussion of the Related Art
In a motor vehicle having a fluid-filled power transmitting device equipped with a lock-up clutch such as a torque converter or fluid coupling incorporating such a lock-up clutch, it is proposed to control the lock-up clutch in a slip control mode (partially slipping or engaging mode) such that an actual amount of slip (slip speed) of the lock-up clutch, namely, a difference between the speeds of a pump impeller and a turbine impeller eventually coincides with a predetermined target slip speed, for the purpose of improving the fuel economy of the vehicle while minimizing the power loss due to slipping of the lock-up clutch. The slip control mode is established when the running condition of the vehicle is in a predetermined slip control area which is intermediate between a fully releasing area in which the lock-up clutch should be held in a fully released state, and a fully engaging area in which the lock-up clutch should be held in a fully engaging state. These fully releasing, fully engaging and slip control areas are defined by suitable parameters (e.g., throttle valve opening and vehicle running speed) indicative of the vehicle running condition.
Generally, the slip control of the lock-up clutch is implemented by a feedback slip control device adapted to control a hydraulic system which uses a hydraulic power source that permits full engagement of the lock-up clutch. The hydraulic system has a releasing oil chamber and an engaging oil chamber which are formed on the opposite sides of a piston of the lock-up clutch so that the piston is movable between fully released and fully engaged positions of the clutch, depending upon a pressure difference of the two oil chambers. The feedback slip control device is arranged to control the pressure difference of the two oil chambers for changing a force acting on the clutch piston to thereby change a friction force of the lock-up clutch and the amount of engagement or slip or the slip speed of the clutch. In this feedback control arrangement, even a small amount of change in the hydraulic pressure difference, that is, even a small amount of change of the slip control output of the slip control device will cause a considerable amount of change of the slip amount or slip speed of the lock-up clutch, since the friction force of the clutch is inherently comparatively unstable with respect to a change in the force acting on the clutch piston. In other words, the amount of slip of the lock-up clutch is sensitive to a change in the value of the slip control output of the slip control device. Thus, the feedback control of the slip amount of the clutch is comparatively unstable. In view of this drawback, a slip control equation used by the slip control device is formulated to determine the slip control output on the basis of a feed-forward control valve, a feedback control value, and a learning control value, as disclosed in JP-A-4-203561.
In some motor vehicle, a traction control device is provided to prevent excessive slipping of drive wheels of the vehicle on a road surface by reducing the amount of opening of the throttle valve. If the vehicle running condition falls in the slip control area of the lock-up clutch as a result of reduction of the throttle valve opening by the traction control device, the slip control device begins to control the lock-up clutch in the slip control mode, whereby the speed difference of the pump and turbine impellers of the torque converter is reduced, and the torque ratio is lowered so that the torque which is transmitted through the torque converter is excessively reduced. When the throttle valve opening is increased to the original value by the traction control device after the slipping tendency of the drive wheels is eliminated, the slip control of the lock-up clutch by the slip control device is terminated, and the speed difference and torque ratio of the torque converter will increase, with a result of an excessive increase in the torque transmitted through the torque converter. Thus, changes in the throttle valve opening by the traction control device cause the slip control device to initiate and terminate the slip control operation of the lock-up clutch. In other words, the slip control of the lock-up clutch is considerably influenced by the operation of the traction control device which controls the amount of opening of the throttle valve. Thus, the mode in which the lock-up clutch is controlled is influenced by the changes in the throttle valve opening effected by the traction control device, and the stability of slip control of the lock-up clutch is deteriorated by the traction control.
Even if the vehicle running condition is kept within the slip control area throughout an operation of the traction control device to control the throttle valve opening, the slip control of the lock-up clutch by the slip control device may be influenced by the traction control device. Described more specifically, the target slip speed of the lock-up clutch used by the slip control device may be determined such that the target slip speed decreases with a decrease in the throttle valve opening. In this case, a decrease in the throttle valve opening by the traction control device results in reducing the target slip speed of the lock-up clutch, causing a decrease in the torque ratio of the torque converter, whereby the torque transmitted through the torque converter is excessively reduced. On the other hand, an increase in the throttle valve opening by the traction control device results in an increase in the target slip speed, causing an increase in the torque ratio and an excessive increase in the torque transmitted through the torque converter. Thus, the control stability of the lock-up clutch is deteriorated by the traction control device. If the target slip speed of the lock-up clutch is determined such that the target slip speed increases with a decrease in the throttle valve opening, a decrease in the throttle valve opening by the traction control device results in increasing the target slip speed, causing an increase in the torque ratio of the torque converter, which offsets a torque control effect of the torque converter. Similarly, the torque control effect is offset by an increase in the throttle valve opening by the traction control device, which results in reducing the target slip speed of the lock-up clutch, causing a decrease in the torque ratio of the torque converter.